Explicity specify bullseye for now

This is because rust-analyzer now requires a more up to date version of
glibc and MS is being slow to update pending some new maintenance plan
microsoft/vscode-dev-containers#1599

Author: nerdondon

.devcontainer/Dockerfile

@@ -1,6 +1,6 @@
 # See here for image contents: https://github.com/microsoft/vscode-dev-containers/tree/v0.191.0/containers/rust/.devcontainer/base.Dockerfile
 
-FROM mcr.microsoft.com/vscode/devcontainers/rust:0-1
+FROM mcr.microsoft.com/vscode/devcontainers/rust:bullseye
 
 # [Optional] Uncomment this section to install additional packages.
 # RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \